This invention generally relates to photolithographically-patterned, out-of-plane micro-device structures for use in integrated circuits, circuit boards and other devices and methods of making such structures.
Copending, coassigned U.S. patent application Ser. No. 09/573,815 filed May 17, 2000, xe2x80x9cPhotolithographically-patterned out-of-plane coil structures and method of making,xe2x80x9d describes various photolithographically produced out-of-plane coil structures and methods of making such structures, which is incorporated herein by reference. These structures are particularly advantageous for forming several practical electrical components for integration on silicon substrates. Out-of-plane inductors, for example, offer several advantages over prior art planar inductors, in that out-of-plane structures minimize eddy currents induced in the underlying substrate and when out-of-plane coils are operated at high frequencies, skin and proximity effects are better controlled.
Out-of-plane coil structures place the coil axis parallel, rather than perpendicular, to the substrate plane. An out-of-plane coil structure includes a substrate and an elastic member having a first anchor portion fixed to the substrate, a loop winding and a second anchor portion connected to the substrate. The second anchor portion and the loop winding are initially fixed to the substrate, but are released from the substrate to become separated from the substrate. An intrinsic stress profile in the elastic member biases the second anchor portion away from the substrate forming the loop winding and causing the second anchor portion to contact the substrate. The resulting coil structure is out-of-the plane of the substrate. The loop winding may also include a plurality of turns.
Various techniques may be used to position the second anchor portion away from the takeoff point of the elastic member, either tangentially or axially. If the second anchor point is positioned tangentially from the takeoff point, the loop winding is generally in the shape of a distorted circle, i.e., the second anchor portion contacts the substrate in the same vertical plane as the first anchor portion. Various techniques may be used to position the second anchor portion tangentially from the takeoff point. For example, a mechanical stop can be fixed to the substrate at the desired location to catch the second anchor point while it is detached from the substrate. Also, the radius of curvature of the elastic member may be varied, such as by adding a load layer onto a portion of the elastic member or by patterning one or more openings or perforations into a portion of the elastic member.
Various techniques can be used to connect the second anchor portion to the substrate. For example, the second anchor portion can be soldered or plated to the substrate. Each anchor portion can be attached to a metal contact pad attached to the substrate, for providing electrical connectivity to other elements in a circuit. Preferably the elastic member is formed of a conductive material. Alternatively, a layer of a conductive metal, such as gold, copper or silver, may be formed on an inner surface, an outer surface, or both surfaces.
An alternate method for forming an out-of-plane coil based on closing half loop pairs of elastic members as disclosed in copending, coassigned U.S. patent application Ser. No. 09/591,262 filed Jun. 9, 2000, xe2x80x9cPhotolithographically-patterned out-of-plane coil structures and method of makingxe2x80x9d is incorporated herein by reference. Upon release the half loop pairs need only to be coarsely aligned to each other and connected together, such as by either plating or soldering. The loop halves need not be the same length. One side could be longer than the other to facilitate the overlap. A different release material may be used under each loop half to release the loop halves sequentially.
One difficulty in creating out-of-plane structures is ensuring that the elastic members used to form the loops are not bunched or entangled by hydrodynamic and surface tension forces when they are being released. It has been observed that aqueous release and drying of the released elastic members causes insufficiently stiff fingers to get pulled around by the air-liquid interface and stick together. The longer and narrower the released elastic members the greater is the problem. A related defect occurs when released elastic members intertwine. Another difficulty is providing enough contact area for the free end of the released elastic member where it makes mechanical contact for subsequent electroforming. A further difficulty is calibrating and maintaining the stress parameters in the metal deposition process in order to keep the diameter of the coil, and as such its inductance, within a few percent tolerance.
The invention describes several methods and structures for improving the yield of out-of-plane structures, including coils and springs. Out-of-plane springs may be used in applications such as flip-chips and probe cards as well as in variable capacitors. An out-of-plane structure, according to one aspect of the invention includes a substrate and at last two spaced-apart elastic members, each elastic member including an anchor portion, a spring and a free end, the anchor portion being fixed to the substrate, the free end being disposed away from the substrate. The free end and the spring are initially fixed to the substrate, but are released from the substrate to become separated from the substrate. An intrinsic stress profile in the elastic member biases the free end away from the substrate forming the spring. A tether layer disposed across (or joining) the elastic members maintains the spaced-apart position of the springs with respect to one another during release and after. This structure produces out-of-plane spring shaped structures or cantilevers. When the springs are made wider, they can be used as the variable plate of a capacitor. When the elastic members are made longer and the intrinsic stress profile increased sufficiently so that the free end contacts the substrate, the elastic members produce loop windings. These structures can be used as inductor coils or transformer coils.
The tether layer is disposed from one elastic member to the other (or joins two elastic members) in order to maintain the relative position of the two members during release. If the structure includes more than two elastic members, the tether layer may extend across or join all members. While in general, for multiple elastic members, a single tether extending across all of the elastic members may be easiest to deposit and pattern, it should be noted that a tether need only join adjacent elastic members, i.e., a plurality of individual tethers, one for each adjacent pair of elastic members, may also be used. Generally, the tether layer will be approximately perpendicular to the length of the elastic members, but may be placed on a diagonal. More than one tether layer may be used for lengthy elastic members. In one embodiment, the elastic members are formed of an electrically conductive material and the tether layer is formed of a non-conductive material. After release and any subsequent processing steps, the tether layers may be removed or remain intact. This embodiment may be easily combined with one or more of the other embodiments described below.
By placing one or more tethers joining the elastic members prior to release, the floppiness problem of very long flexible released elastic members is essentially eliminated. The tethers on the elastic members release along with the elastic members, and maintain spacing and separation of the individual released elastic members. The tethers may be made sufficiently narrow or perforated to ensure that the release etch releases the tethers along with the released elastic members (to promote release, if the released elastic members are perforated, the perforations will generally extend through to the release layer). The tethers maintain uniform array spacing and prevent the released elastic members from touching or entangling. The ensemble of tethered released elastic members behaves like an effectively stiffer structure. The tethers actually constrain the undesired flexures of the released elastic members relative to one another, while leaving the curling behavior of the released elastic members largely unchanged. There can be a loading effect due to the tether that effectively changes the radius of the released elastic member relative to what it would be without the tether. Design considerations can allow for this effect.
The tether structure has utility anywhere that a floppy out-of-plane structure would otherwise suffer from unconstrained flexure, including applications for coils and out-of-plane spring cantilevers such as flip chip packages and probe cards. For example, tethers can be used to hold the springs in uniform registration during release and a subsequent plating step that substantially stiffens the structures.
Rate of release of the elastic member used to form an out-of-plane structure is controlled in another embodiment of the invention. An out-of-plane structure includes a substrate and an elastic member including a first anchor portion fixed to the substrate, a spring and a free end. The elastic member further includes a plurality of perforations unequally spaced apart from one another in the spring. The free end and the spring are initially fixed to the substrate, but are released from the substrate to become separated from the substrate. An intrinsic stress profile in the elastic member biases the free end away from the substrate forming the spring. On release of the free end, the perforations in the elastic member control a rate of release of the elastic member from the substrate. Undercut etching occurs both under the edges of the elastic member and the edges of the perforations. Perforations may also be used to control release of elastic members which form loop windings when the free end contacts the substrate.
Perforations in the material of the elastic member allow etchant to have greater access to the interior portions of the member segment, and thereby allow the elastic member to release faster with less undercut etching at the member base. By grading the perforation density, or having the perforations unequally spaced along the member, the elastic member then releases from the substrate in a controlled fashion starting with the tip or free end, and progressing toward the base or anchor portion. This can be important because of the large amount of elastic energy that is stored in the elastic member. If the release rate of the energy is too rapid, the elastic member can reach enough speed to entangle with other elastic members or break. Gradual, controlled release of the elastic member allows mechanical damping enough time to limit the total kinetic energy of the elastic member to a non-destructive level.
The perforations, if made small enough, may be closed off after elastic member release by electroforming them with a suitable metal such as copper. If the free end is attached by electroforming (to form a loop structure, for example), the perforations can be closed off in the same plating bath during the released elastic member attachment process.
Controlling the radius of curvature in both spring structures and coil structures is often required by design considerations. One of the ways of controlling the radius of curvature of the coil structures is to deposit a load layer on the elastic member prior to release of the elastic member. In accordance with another embodiment of the invention, further control can be achieved by depositing the load layer using a reflow material, i.e., any material that softens at a process compatible temperature. An example of a suitable reflow material is photoresist, but other suitable reflow materials may also be used. The load layer of reflow material can be introduced in the same masking step that creates the release window, or it can be introduced in a separate step. One desirable feature of using a photoresist load layer is that the loading effect of the resist can be gradually changed with heat and clean up is easily accomplished with plasma ashing.
In accordance with another embodiment of the invention, the radius of curvature may be further controlled by depositing a multilayer load layer. By depositing a multilayer load layer, the radius of curvature may be controlled by selectively etching away individual layers of the load layer.
In accordance with another embodiment of the invention, an out-of-plane coil structure, may be formed by joining two elastic members in mid-air. To facilitate latching of the two elastic members, one free end includes an elongated tip and the other free end includes a tip having a groove for receiving the elongated tip. The two free ends can be easily connected by soldering or plating.
In accordance with another embodiment of the invention an out-of-plane coil structure may be formed by joining two elastic members in which one side may lift away from the substrate by only a small amount, providing a tab that acts as a mechanical stop and/or alignment structure for the other elastic member in the pair. In addition, the longer member of the pair of elastic members may contact both the tab and the substrate.
To facilitate formation of out-of-plane coil structures an extended xe2x80x9cYxe2x80x9d and xe2x80x9cUxe2x80x9d base pad structure may be used. An elongated tip is formed at the free of the elastic member before release. The anchor portion is formed in the shape of an inverted xe2x80x9cUxe2x80x9d permitting the extended tail of the xe2x80x9cYxe2x80x9d shaped contact pad for an adjacent elastic member to be positioned within the inverted xe2x80x9cUxe2x80x9d. Thus after release the tip will bisect the extended tail portion of the Y facilitating coil completion.